The present invention relates generally to wireless communications, and more particularly, to a hybrid automatic-repeat-request (HARQ) reception method with channel state information.
HARQ systems combine ARQ and error control coding to achieve high throughput and high reliability. Unlike ARQ systems where packets decoded previously in error are simply discarded, in HARQ systems all the received signals are combined to enhance performance. Chase-combining HARQ (CC HARQ) is a simple HARQ scheme where the transmitter transmits the same codeword (using the same modulation and coding scheme) upon receiving a decoding error message from the receiver.
Referring to the diagram in FIG. 1, we consider a multiple access channel where K users 11-13 communicate with a base station 10 equipped with multiple antennas. The communication system in which the wireless users and base station communicate employs Chase-combining Hybrid-Automatic-Repeat-Request (CC-HARQ), which is a simple HARQ scheme where each user re-transmits its encoded signal, using its original (first transmission) modulation and coding scheme) upon receiving a decoding error message (NACK) from the receiver of the base station 10. An optimal Chase-combiner utilizes received sufficient statistics from all the received signal transmissions to perform pre-demodulation combining in every time slot. It is not suitable for implementation, particularly, in multi-user systems due to its high memory requirement and complexity.
A conventional receiver that is used for chase combining, implements post-demodulation bit-level combining in which the log-likelihood ratios (LLRs) for each coded bit are added up across all received transmissions. An optimal chase combining receiver is also known in the prior art and this receiver implements pre-equalization symbol-level combining. The optimal receiver can offer significant gains over the conventional one since the latter does not fully exploit the available received observations. However, in practice the conventional one is preferred because the optimal receiver requires prohibitive complexity and memory consumption.
Accordingly, there is a need for a Chase-combining hybrid automatic-repeat-request (CC-HARQ) receiver technique that achieves the performance of the optimal receiver technique, but with the memory requirement and complexity of the conventional receiver.